Plastic dental compositions are increasingly being used for dental restoration in the dental sector. Such plastic dental compositions usually include a matrix of organic resins and various inorganic fillers. Inorganic fillers predominantly comprise powders of glasses, (glass-) ceramics, quartz or other crystalline substances (e.g. YbF3), sol-gel materials or aerosils, which are added to the plastic composition as filling material.
The use of plastic dental compositions is intended to avoid possible harmful side-effects of amalgam and to achieve an improved aesthetic impression. Depending on the plastic dental compositions selected, they can be used for different dental restoration purposes, for example, for tooth fillings, as well as for securing parts, such as crowns, bridges and inlays, onlays etc.
The filling material per se is intended to minimize the shrinkage caused by polymerization of the resin matrix during curing. For example, if there is a strong adhesion between the tooth wall and filling, excessive polymerization shrinkage can lead to the tooth wall breaking. If the adhesion is inadequate, excessive polymerization shrinkage may result in the formation of peripheral gaps between the tooth wall and filling, which can promote secondary caries. Furthermore, certain physical and chemical demands are imposed on the fillers.
It is desirable to process the filling material to form powders that are as fine as possible. The finer the powder, the more homogenous the appearance of the filling. At the same time, the polishing properties of the filling are improved, which in addition to reducing the surface area available for attack also leads to improved resistance to abrasion and therefore to a longer-lasting filling. To enable the powders to be processed successfully, it is also desirable for the powders not to agglomerate. This undesirable effect tends to occur with filling materials produced using sol-gel processes.
Furthermore, it is advantageous if filler particles are coated or at least partially coated with functionalized silane, since this facilitates formulation of dental compositions and improves the mechanical properties.
Furthermore, the refractive index and color of the entire plastic dental composition, including fillers, should be as well matched as possible to the natural tooth material, so that it is as indistinguishable as possible from the surrounding, healthy tooth material. The grain size of the pulverized filler being as small as possible also helps to achieve this aesthetic criterion.
It is also important for the thermal expansion of the plastic dental composition in the typical range of use, i.e. usually between −30° C. and +70° C., to be matched to that of the natural tooth material in order to ensure that dental restoration measures are sufficiently able to withstand temperature changes. Excessively high stresses caused by temperature changes also can cause formation of gaps between plastic dental compositions and the surrounding tooth material, which in turn can form sites of attack for secondary caries. In general, fillers with the lowest possible coefficient of thermal expansion are used to compensate for the high thermal expansion of the resin matrix.
Good chemical resistance of the fillers with respect to acids, alkalis and water and good mechanical stability under load, such as, for example, during movement produced by chewing, can also contribute to a long service life for dental restoration measures.
Furthermore, for the treatment of patients, it is imperative that dental restoration measures can be seen in an X-ray image. Since the resin matrix itself is generally invisible in an X-ray image, the fillers must provide the required X-ray absorption. A filler of this type which provides sufficient absorption of X-radiation is described as X-ray opaque. Constituents of fillers, for example, certain components of a glass, or other substances, are generally responsible for X-ray opacity. Such substances are often referred to as X-ray opacifiers. A standard X-ray opacifier is YbF3, which can be added to the filler in crystalline, milled form.
According to International Standard DIN ISO 4049, the X-ray opacity of dental glasses or materials is quoted in relation to the X-ray absorption of aluminum as aluminum equivalent thickness (ALET). The ALET is the thickness of an aluminum sample which has the same absorption as a 2 mm-thick sample of the material to be tested. An ALET of 200% therefore means that a small glass plate having plane-parallel surfaces and a thickness of 2 mm produces the same X-ray attenuation as a small aluminum plate with a thickness of 4 mm. Analogously, an ALET of 150% means that a small glass plate having plane-parallel surfaces and a thickness of 2 mm produces the same X-ray attenuation as a small aluminum plate with a thickness of 3 mm.
Because plastic dental compositions in use are usually introduced into cavities from cartridges and then modeled in the cavities, such compositions should be at least somewhat thixotropic in the uncured state. This means that viscosity decreases when pressure is exerted, while it is dimensionally stable without the action of pressure.
Among plastic dental compositions, a distinction also should be drawn between dental cements and composites. In the case of dental cements, also known as glass ionomer cements, the chemical reaction of fillers with the resin matrix leads to curing of the dental composition, and consequently the curing properties of the dental composition. Thus, their workability is influenced by the reactivity of the fillers. This often involves a setting process which is preceded by a radical surface curing, for example, under the action of UV light. Composites, also referred to as filling composites contain, by contrast, fillers which are as chemically inert as possible, since their curing properties are determined by constituents of the resin matrix itself and a chemical reaction of the fillers often disrupts this.
Because glasses, due to their different compositions, represent a class of materials with a wide range of properties, they are often used as fillers for plastic dental compositions. Other applications as dental material, either in pure form or as a component of a material mixture, are also possible, for example, for inlays, onlays, facing material for crowns and bridges, material for artificial teeth or other material for prosthetic, preservative and/or preventive dental treatment. Glasses of this type used as dental material are generally referred to as dental glasses.
In addition to the dental glass properties described above, it is also desirable for this glass to be free from barium and/or barium oxide (BaO), which are classified as harmful to health, and also from lead and/or lead oxide (PbO) and from other barium and lead compounds.
In addition, it is also desirable for a component of dental glasses to be zirconium oxide (ZrO2). ZrO2 is a widely-used material in the technical fields of dentistry and optics. ZrO2 is readily biocompatible and is distinguished by its insensitivity to temperature fluctuations. It is used in a wide variety of dental supplies in the form of crowns, bridges, inlays, attachment work and implants.
Dental glasses therefore represent glasses of particularly high quality. Glasses of this type also can be used in optical applications, particularly if such applications benefit from the X-ray opacity of the glass. Since X-ray opacity means that the glass absorbs electromagnetic radiation in the region of the X-ray spectrum, corresponding glasses simultaneously act as filters for X-radiation. Sensitive electronic components can be damaged by X-radiation. In the case of electronic image sensors, for example, the passage of an X-ray quantum may damage the corresponding region of the sensor or result in an undesirable sensor signal which can be perceived, for example, as an image disturbance and/or disturbing pixels. For specific applications it is therefore necessary, or at least advantageous, to protect electronic components against X-radiation by using corresponding glasses to filter said components out from the spectrum of the incident radiation.
A number of dental glasses and optical glasses are known from the prior art.
For example, DE102004026433 describes glass or glass powders having a mean particle size of <1 μm. These glasses may be used, inter alia, as dental glasses. The compositions described therein, however, do not point to the special features of glasses according to the present invention.
DE3524605 C2 describes fluorine-containing glasses for optical waveguides produced by ion exchange. Fluorine, however, is undesirable in dental glasses. Glasses described in this reference also may contain 0-2 mol % Cs2O and X-ray opacifiers, such as BaO, SrO, PbO, present in an amount of 0 to 1 mol %. Such compositions are not capable of achieving high X-ray opacity. A K2O content of 6-18 mol % also means that good resistance is not achieved.
DE4029230 describes a dental material having a polymerizable binder, an amorphous filler such as SiO2 and glass or glass ceramic, and an X-ray opacifier. Here, X-ray opacity requires the addition of a so-called X-ray opacifier such as ytterbium trifluoride.
DE60315684 describes glass filler material for use in dental composites and dental restoration. The total content of the alkali metals, at 0.05 to 4 mol %, is too low to ensure sufficient melting to achieve high throughput. A high throughput is important primarily for economic operation.
DE102005019958 describes glasses for use as flash lamp glass. Flash lamp glass is preferably free from Cs2O and alkali metals, but as a result contains alkaline-earth metals (the total content of MgO, CaO, SrO, BaO being 2-30% by weight). However, even small amounts of CaO may affect mechanical properties, such as, for example, the Vickers hardness. Increased Vickers hardness is disadvantageous in the milling process since the milling bodies are subjected to increased abrasion and the process takes longer.
DE102006012116 A1 describes glass fiber cables for data transmission. The glasses described therein are X-ray opaque only to a certain degree. An essential component for the X-ray opacity is described merely as <2% by weight La2O3. However, in addition to Cs2O, an La2O3 content of higher than 2% by weight is required.
U.S. Pat. No. 3,529,946 describes a process for curing the surface by ion exchange. The glass suitable for this purpose has to contain, inter alia, TiO2, which is not present in the glass according to the invention. TiO2 shifts the UV edge of the glass to the longer-wave region and thus shifts the color locus of the glass into undesirable regions. Dental glasses should be white. In addition, Li2O must be present in an amount of 2.5 to 4% by weight in order to ensure ion exchange. This effect, however, is undesirable in dental glasses since glasses should be stable with respect to any leaching. Li2O is quickly leached out of the glass and, if dental material is present, can reduce the resistance thereof. Such glasses are furthermore destabilized by the leaching-out itself. Transparency also can be adversely affected. Thus, leaching-out should also be avoided for optical glasses.
U.S. Pat. No. 5,132,254 describes a composite material. The glass or glass ceramic matrix described therein must contain >25% alkaline-earth metal oxides. For reasons mentioned above, alkaline-earth metal oxides are not desirable for certain applications. See supra, DE102005019958.
Features common to the glasses mentioned in the prior art are that they either (1) have low weathering resistance and/or (2) are not X-ray opaque and/or (3) are often difficult or expensive to produce and/or (4) contain components which are harmful to the environment and/or to health.